The present invention is directed to electrical contacts that comprise spaced particles embedded into the surface of conductors in which the particles have been kinetically sprayed onto the conductors with sufficient energy to form direct mechanical bonds between the particles and the conductors in a pre-selected location and particle number density that promotes high surface-to-surface contact and reduced contact resistance between the conductors. The method of making such electrical contacts is also provided.
U.S. Pat. No. 6,139,913, xe2x80x9cKinetic Spray Coating Method and Apparatus,xe2x80x9d is incorporated by reference herein.
Most electrical contacts are copper or copper alloy conductors with a tin-plated surface layer. The tin surface layer is a single continuous layer directly bonded to a clean non-oxidized copper substrate in order to promote maximum conductance between conductors while limiting resistance from the tin-copper metallic bond. Tin is used as a surface layer since it is substantially softer than copper and may be recurrently wiped to provide a fresh de-oxidized surface for metal-to-metal connection between conductors.
Electrical contacts have been traditionally made by electroplating a layer of tin to copper substrates followed by stamping out individual conductors. The copper substrates must be cleaned prior to placement in the electroplating bath to remove any oxidized surface layers that may otherwise create additional electrical resistance. The substrates are coated to a thickness of about 3 to 5 microns of tin.
Because most electrical contacts undergo repeated connections and reconnections, increasing the thickness of the tin surface layer correlates well with the longevity and durability of the contact. However, due to processing limitations and increased frictional properties, the threshold thickness for electroplating tin onto copper is about 5 microns.
While it may be possible to use other available coating methods to increase coating thickness, methods that rely on melting and/or depositing the tin in a molten state are undesirable because, unless conducted in the absence of oxygen, they will introduce significant oxidation into the tin surface layer. Also, due to the increased costs of use, such methods are not practical.
One of the main problems with present electrical contacts is debris build-up due to fretting on the contact surface. With relative movement of mated electrical contacts, a small portion of the oxidized surface layer is rubbed away to expose a fresh electrical connection surface. The portion rubbed away usually does not flake off, but instead remains adjacent to the contact point and begins to create a build-up of oxidized debris. It is well known that this oxidized debris becomes a source for additional resistance and degradation of the contact""s conductance.
Prior to the present invention, removal of this debris has been impractical. In the prior art, the solution has been to provide continuous layer coatings that have been believed to result in maximum surface area for conductance.
A new technique for producing coatings by kinetic spray, or cold gas dynamic spray, was recently reported in an article by T. H. Van Steenkiste et al., entitled xe2x80x9cKinetic Spray Coatings,xe2x80x9d published in Surface and Coatings Technology, vol. 111, pages 62-71, Jan. 10, 1999. The article discusses producing continuous layer coatings having low porosity, high adhesion, low oxide content and low thermal stress. The article describes coatings being produced by entraining metal powders in an accelerated air stream and projecting them against a target substrate. It was found that the particles that formed the coating did not melt or thermally soften prior to impingement onto the substrate.
This work improved upon earlier work by Alkimov et al. as disclosed in U.S. Pat. No. 5,302,414, issued Apr. 12, 1994. Alkimov et al. disclosed producing dense continuous layer coatings with powder particles having a particle size of from 1 to 50 microns using a supersonic spray.
The Van Steenkiste article reported on work conducted by the National Center for Manufacturing Sciences (NCMS) to improve on the earlier Alkimov process and apparatus. Van Steenkiste et al. demonstrated that Alkimov""s apparatus and process could be modified to produce kinetic spray coatings using particle sizes of greater than 50 microns and up to about 106 microns.
This modified process and apparatus for producing such larger particle size kinetic spray continuous layer coatings is disclosed in U.S. Pat. No. 6,139,913, Van Steenkiste et al., that issued on Oct. 31, 2000. The process and apparatus provide for heating a high pressure air flow up to about 650xc2x0 C. and accelerating it with entrained particles through a de Laval-type nozzle to an exit velocity of between about 300 m/s (meters per second) to about 1000 m/s. The thus accelerated particles are directed toward and impact upon a target substrate with sufficient kinetic energy to impinge the particles to the surface of the substrate. The temperatures and pressures used are sufficiently lower than that necessary to cause particle melting or thermal softening of the selected particle. Therefore, no phase transition occurs in the particles prior to impingement.
The present invention is directed to kinetic spraying electrically conductive materials onto conductive substrates. More particularly, the present invention is directed to electrical contacts that comprise spaced electrically conductive particles embedded into the surface of conductors in which the particles have been kinetically sprayed onto the conductors with sufficient energy to form direct mechanical bonds between the particles and the conductors in a pre-selected location and particle number density that promotes high surface-to-surface contact and reduced contact resistance between the conductors. The particle number density, as used herein, defines the quantity of spaced particles deposited within a selected location.
Utilizing the apparatus disclosed in U.S. Pat. No. 6,139,913, the teachings of which are incorporated herein by reference, it was recognized that thick continuous layer coatings could be produced on conductive substrates in the production of electrical contacts. Such thick coatings are practical due to the mechanical bonds that are formed by impact impingement of the particles onto the substrate. These thicker continuous layer coatings are beneficial in producing electrical contacts since they provide low porosity, low oxide, low residual stress coatings that result in electrical contacts having greater longevity and durability.
When the feed rate of the particles into the gas stream is reduced, it is difficult to maintain a uniform output of particles necessary to form a continuous layer. The production of a continuous layer of particles is even more problematic if the substrate is moved across the nozzle or vice versa.
The present inventors used this process to embed a large number of spaced apart particles in the surface of conductors to provide multiple contact points that are particularly useful for electrical contacts. A large number of spaced particles embedded in the surface of the conductors provide a structure having a surface layer with a plurality of particles forming ridges and valleys. Each embedded particle defines a ridge, and the space in between particles defines a valley. The ridges provide multiple contact points for conductance while the spaces provide multiple avenues for the removal of debris produced from repeated fretting. Thus the discontinuous nature of the particle coating caused by the method of application leads to an electrically conductive contact that can with stand repeated fretting, as discussed further below.
In addition, the present invention provides the means for controlling the location of deposition of kinetic sprayed particles and the particle number density deposited in that location on the conductive substrate by simply controlling the feed rate of particles into the gas stream and the traverse speed of the substrate across the apparatus and/or nozzle. By doing so, the spray of conductive materials is controlled so that particles are only deposited on those portions that are to be stamped out as conductors.
This provides a tremendous advantage in processing. It substantially reduces waste of the conductive particles and aids in the reuse of substrate materials. Furthermore, there are no plating bath waste products or associated disposal costs.
In a typical coating procedure it is necessary to pre-clean the surface that is to be coated to remove the oxide layer, the present process eliminates this step. The impact of the initial kinetic sprayed particles on the surface is sufficiently forceful to fracture any oxide layer on the surface. The subsequent particles striking the now cleaned surface stick. As a result, electrical contacts produced by kinetic spraying spaced electrically conductive particles are particularly useful.
The present invention provides that particles can be kinetic sprayed onto conductors with sufficient energy to form direct mechanical bonds between the particles and the conductors in a pre-selected location and particle number density that promotes high surface-to-surface contact between the conductors with reduced contact resistance.